We studied the early phase of herpetic endophthalmitis to clarify the process of necrotizing retinitis by herpes simplex virus type-1 (HSV-1) and viral localization in the retina. A clone, isolated from HSV-1 Miyama for its high yield on Vero cells, was inoculated on mono-layers of Vero cells. Two days after inoculation, the cells were frozen-and-thawed once and were centrifuged. The supernatant was diluted with Hanks' balanced saline solution. The viral suspension 1.5 x 10(5) pfu/ml) was injected intravitreally to colored rabbits for 0.1 ml. All rabbits showed exudative iritis from 1 to 2 days after inoculation, vitreal opacities from 2 to 4 days and retinal exudates from 2 to 6 days. These exudates initially considered of necrotic materials of the inner nuclear layer. These fused with one another gradually and formed massive exudates which spread to the whole retinal layer, causing retinal necrosis. Viral particles were found in the nuclei of the inner nuclear layer at first, and later in the nuclei of Müller cells, ganglion cells and axon of the nerve fiber. These findings indicated that retinal lesion occurred primarily in viral replication of the inner nuclear layer.